1. Field of the Invention
The present invention relates to optical waveguide, and more particularly to, a structure of optical grating slab waveguide having a light waveguiding layer with linear refractive index disposed on another waveguiding layer having nonlinear refractive index that may be used as active optical interconnection, optical switching devices, and wavelength selective coupling devices of bidirectional optical coupler.
2. Description of the Related Art
Over the past decades there has been considerable interest in optical grating arrays disposed on a slab waveguide. A conventional structure of grating slab waveguide is consisted of the grating structures and single-layer-waveguiding film stacked on substrate. The waveguide film has either linear or nonlinear coefficient of refractive index. The optical grating structures disposed on the waveguide film have been widely used to optical grating coupler, and optical interconnection. In particular, the nonlinear slab waveguide operates as a power dependent grating coupler or all optical beam scanner. Also, grating assisted directional coupler(GADC) with linear or nonlinear slab waveguide has been greatly investigated because this structure can be applied to wavelength selective coupler of wavelength division multiplexing (WDM).
Focusing grating coupler array based on slab waveguide with waveguiding layer having a linear refractive index is disclosed in Applied optics, Vol. 34, NO. 26, page 5913-5919, September 1995, by Song and Lee. In the operation of the focusing grating coupler array, the input signal beams guided into the slab waveguide are collimated by the guide-lenses and they propagate in the waveguiding film. The propagating light is diffracted at each point of the chirped grating toward the focal point with the diffraction angle q. The wavelength of diffracted light is a function of a diffraction angle q. The guided wave decays exponentially with the propagation in the waveguiding film due to the power leakage resulting from the out coupling to the radiation modes. After the guided wave passes through a focusing grating coupler array with length of L, the amplitude is decayed by the factor of EQU A(L)=e.sup.-.alpha.L ( 1)
Where a is radiation decay coefficient, obtained by EQU .alpha.=(1-t)/(4T tan .theta..sub.0) (2)
Where .theta..sub.0 is propagation angle of the light, T is effective thickness of the waveguiding film, and t is an efficiency of zeroth order diffraction.
A multiplexer or demultiplexer with wavelength selective coupling devices consisted of conventional slab waveguide structure suffer from the drawbacks of optical power loss due to out coupling to the radiation mode.